A Pd complex, cis-[Pd(C6 F5 )2 (THF)2 ] (1), is proposed as a useful touchstone for direct and simple experimental measurement of the relative ability of ancillary ligands to induce C-C coupling. Interestingly, 1 is also a good alternative to other precatalysts used to produce Pd0 L. Complex 1 ranks the coupling ability of some popular ligands in the order Pt Bu3 >o-TolPEWO-F≈tBuXPhos>P(C6 F5 )3 ≈PhPEWO-F>P(o-Tol)3 ≈THF≈tBuBrettPhos≫Xantphos≈PhPEWO-H≫PPh3 according to their initial coupling rates, whereas their efficiency, depending on competitive hydrolysis, is ranked tBuXPhos≈Pt Bu3 ≈o-TolPEWO-F>PhPEWO-F>P(C6 F5 )3 ≫tBuBrettPhos>THF≈P(o-Tol)3 >Xantphos>PhPEWO-H≫PPh3 . This "meter" also detects some other possible virtues or complications of ligands such as tBuXPhos or tBuBrettPhos.
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